Over the last year the Center has become more involved in the running of mammalian preparations needing accurate temperature control. This is not a minor problem. Not only does one need to elevate the experimental environment to 37oC but we must also assess whether our systems, which seldom if ever, operate under these conditions, will remain stable. A major concern was the generation of temperature differentials which we have already observed to cause significant problems with the old voltage self-referencing probes. With Seris (ion-selective) or Serp (polarographic) electrodes, mixing of the boundary layer produces a catastrophic failure in the technique and drove the need to stabilize the temperature gradients. Two approaches have 1. Commercially available temperature controllers. Trials have been conducted with two devices, one supplied by Carl Zeiss and the other by Medical Systems. The latter has proved most effective allowing the passage of temperature equilibrated gas over the air saline boundary, reducing evaporative loss and temperature profiles. In addition, this system permits thermostatic regulation of either the plate or the bath. Our most recent experiments on mammalian oocytes and embryos with epidermal growth factor stimulation of calcium flux have utilized this device. One possible problem with plate temperature controllers is the possible damage the external gradients might cause to high-resolution optics. Our alternative approach solves this problem. 2. Temperature regulated chambers. In order to elevate the operating temperature to 37oC while minimizing the impact of convective mixing we have housed a dual Seris and Serp system inside a home made environmental chamber. The housing consists of a steel-box which doubles as a Faraday cage. It is insulated on the outside. In this way heat loss from the box is minimized while the walls, air-table and microscope act as a large thermal capacitor damping temperature fluctuations within the chamber. The internal temperature is regulated via an Omega process controller (CN3000). The solid state relays switch power from a variac to a rectifier and filter, and then to a pair of 1500 Watt heating elements. The variacs can be set for a maximum temperature, providing a safety feature. Temperature in the chamber is measured using a suspended themistor (Omega 400 Series). Taking advantage of the new Windows capability we have written a program that operates in the background of the Ion View software controlling the Serp and Seris electrode data collection. This program records the chamber temperature at a user specified interval (default is every 10s). The program can also be used to access and adjust the algorithm regulating the process controller.